This invention relates to an uninterruptible power system, and more particularly to an uninterruptible power system of the normally commercial power feed type adapted to carry out electric supply to a load through an AC switch when a commercial power supply is in a normal or silent condition and turn off the AC switch when power stoppage or a power failure occurs in the commercial power supply, to thereby convert DC power previously stored in a storage battery into AC power by means of a power converter, leading to feeding of the AC power to the load, resulting in ensuring power electric supply without interruption.
A conventional control circuit for an uninterruptible power system is disclosed in Japanese Patent Application Laid-Open Publication No. 51735/1996, Japanese Patent Application Laid-Open Publication No. 56087/1997 and Japanese Patent Application Laid-Open Publication No. 14251/1998. The conventional control circuit disclosed is so constructed that a command having a current control command and a voltage control command added to each other is inputted to a PWM comparator of a power converter with an active filter function to prepare a gate signal for the power converter. The conventional control circuit is adapted to output a current control command to the uninterruptible power system supposing that a commercial power supply is normal or silent unless a power failure detection circuit detects a power failure of the commercial power supply, even when the commercial power supply is actually reduced in voltage below a predetermined level.
Thus, the conventional uninterruptible power system is fed with a current control command supposing that no abnormality occurs unless power failure of the commercial power supply is detected, even when any abnormality in voltage such as a voltage reduction actually occurs in the commercial power supply. This causes strain at a large magnitude to occur in an output voltage thereof fed to a load due to a current which intends to flow from the power converter toward the commercial power supply.
Also, the conventional uninterruptible power system often includes a DC/DC converter arranged between the power converter and a storage battery. The DC/DC converter acts to frequently operate when any abnormality in voltage such as a voltage reduction occurs in the commercial power supply, to thereby feed DC power from the storage battery to the power converter. Unfortunately, this causes the storage battery to be deteriorated in durability or operating life.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide an uninterruptible power system which is capable of significantly reducing strain in an output voltage thereof.
It is another object of the present invention to provide an uninterruptible power system which is capable of enhancing durability of a storage battery or an operating life thereof.
In accordance with the present invention, an uninterruptible power system is provided. The uninterruptible power system includes an abnormal voltage detection circuit for detecting voltage reduction abnormality of a commercial power supply, an AC switch turned on to permit AC power to be fed from the commercial power supply to a load when the commercial power supply is in a normal condition and turned off when the commercial power supply falls into an abnormal condition, a storage battery, and a power converter of the voltage control type arranged between the AC switch and the storage battery. The power converter carries out power rectifying operation of converting AC power of the commercial power supply into DC power to charge the storage battery by the DC power and active filter operation of flowing a current for compensating a harmonic current and/or a reactive current flowing to the load when the commercial power supply is in a normal condition. The power converter carries out power inverting operation of inverting DC power into AC power to feed the AC power to the load mainly using the storage battery as a power supply therefor when the commercial power supply is in a condition other than the silent condition. The uninterruptible power system further includes a control command generation means for outputting a control command for feeding AC power to the load without a power failure to the power converter. The control command generation means includes a voltage control system for outputting a voltage control command which permits the power converter to carry out the power inverting operation and a current control system for outputting a current control command which permits the power converter to carry out the power rectifying operation and active filter operation. The uninterruptible power system further includes a reference voltage generating circuit for generating a reference sinusoidal voltage synchronized with the commercial power supply.
In the present invention generally constructed as described above, the control command generation means is configured so as to output the current control command and voltage control command to the power converter while adding them to each other. The current control system is constructed so as to provide an input current command value on the basis of a multiplied value obtained by multiplication between a differential voltage between a DC command voltage commanded by a DC voltage command and a DC-side voltage of the power converter and an output of the reference voltage generating forming circuit, and to thereby output the current control command on the basis of a value obtained by subtracting a load current value from the input current command value. The current control system is constructed so as to use an AC current flowing through the AC switch or a value corresponding to the AC current as the input current command value when the abnormal voltage detection circuit carries out detection of voltage reduction abnormality in the commercial power supply. The value obtained by subtracting the load current flowing to the load from the input current command value acts as a command value indicating a current flowing through the power converter. A value obtained by subtracting a value of a current flowing to an AC-side of the power converter from the command value acts as a corrected value of a current command of the power converter or a current control command. The term xe2x80x9cvalue corresponding to AC currentxe2x80x9d as used herein is intended to mean a value proportional to the AC current in predetermined relationship. When voltage reduction abnormality occurs in the commercial power supply, the AC current flowing through the AC switch is gradually decreased toward zero with the voltage reduction. Thus, use of the AC current or the value corresponding to the AC current as the input current command value permits a reduction in current control command with the voltage reduction, to thereby reduce strain in the output voltage as compared with the prior art.
The current control system may be configured so as to output an added value obtained by addition between a value obtained by increasing a difference between an AC current flowing through the AC switch and the multiplied value by constant times and the AC current flowing through the AC switch as the input current command value. This permits the output to further approach a sinusoidal waveform.
When PQ operation is used in the current control system, the current control system may be configured so as to provide a momentary load-side effective power value corresponding to momentary effective power fed to the load on the basis of an output of the reference voltage forming circuit and the load current value and provide a momentary input-side effective power command value which permits an input current to have a sinusoidal waveform by removing an AC component from the momentary load-side effective power command value, to thereby output the current control command on the basis of a value obtained by subtracting the momentary load-side effective power value from the momentary input-side effective power command value. Also, the current control system, when the abnormal voltage detection circuit detects voltage reduction abnormality in the commercial power supply, may be configured so as to use a momentary effective power value obtained from an output of the reference voltage generating circuit and an AC current flowing through the AC switch as the momentary input-side effective power command value. In the PQ operation as well, use of the momentary effective power value obtained from the output of the reference voltage forming circuit and the AC current flowing through the AC switch as the momentary input-side effective power command value likewise leads to a reduction in current control command with the voltage reduction, to thereby reduce strain in the output voltage as compared with the prior art.
The uninterruptible power system of the present invention may further include a power failure detection circuit for detecting a power failure of the commercial power supply and a DC/DC converter arranged between the power converter and the storage battery. The DC/DC converter may be configured so as to feed DC power from the storage battery to the power converter during a period of time for which the power failure detection circuit detects a power failure of the commercial power supply. Such configuration eliminates waste discharge of the storage battery, to thereby enhance durability thereof. The DC/DC converter may include a capacitor connected in parallel to the storage battery. In this instance, the power converter may feed an AC current to the load using the capacitor as a power supply therefor during a period of time extending from detection of a voltage reduction of the commercial power supply by the abnormal voltage detection circuit to detection of a power failure of the commercial power supply by the power failure detection circuit. This permits discharge of the capacitor rather than the storage battery to feed AC power to the load. In general, repeating of charging and discharge of a capacitor does not cause a rapid deterioration in life thereof as compared with a deterioration in life of a storage battery due to frequent discharge thereof. The power failure detection circuit may be constructed in any desired manner. For example, a power failure of the commercial power supply may be judged to occur when a period of time for which an average of an output of the commercial power supply is equal to or lower than a predetermined level is equal to or exceeds a predetermined period of time. In this instance, the predetermined period of time may be set to be shorter than a period of time required for a voltage of the capacitor to be reduced to a predetermined level or below. Alternatively, the power failure detection circuit may be configured so as to detect a power failure of the commercial power supply when a voltage across the capacitor is reduced to a level equal to or below a predetermined voltage level. This permits smooth changing-over from discharge of the capacitor to that of the storage battery without causing over-discharge of the capacitor. Such construction or technical idea may be solely employed.